4 Combination of Elevated CO 2 Levels and Soil Contaminants’ Stress ... 77
Interestingly, CAT activity was lower in 2 mg kg−1 Cd treatment group at panicle-
initiation growth stage (Fig. 4.4a, b). In all Cd treatment groups, the activities of
APX, POD, and SOD in leaves of wheat grown on FACE plots were higher than that
in leaves of wheat grown on ambient plots (Fig. 4.4c, h).
Studies have shown that metals such as Cu and Cd exhibit the ability to produce
reactive oxygen species (ROS) such as superoxide ion, hydrogen peroxide, and hy-
droxyl radical, resulting in lipid peroxidation (Stohs and Bagchi 1995 ; Skorzyn-
ska-Polit et al. 2006 ; Schraudner et al. 1997 ). The excessive production of ROS
in chloroplasts of plants has proven to be the main cause of oxidative damage in
leaves (Foyer and Noctor 2003 ). Plants have evolved an antioxidant defense system
including enzyme system and nonenzymatic system to avoid damage caused by
ROS. The antioxidant defense system plays an important role in the ROS removal
and protective defense reaction (Hernandez et al. 2001 ). SOD is a major scaven-
ger of superoxide ion, its enzymatic action results in the formation of hydrogen
Fig. 4.2 Changes in the activities of CAT (a), APX (b), POD (c), SOD (d) in the leaves of rice
at mid-tillering growth stage. Rice plants grown in soil with 0, 0.5, or 2 mg kg−1 Cd, under either
ambient CO 2 levels or elevated CO 2 levels ( FACE). Values represent means ± SD. An asterisk
indicates a significant difference between FACE and ambient conditions ( p < 0.05). CAT cata-
lase, APX ascorbate peroxidase, POD peroxidase, SOD superoxide dismutase, FACE free-air CO 2
enrichment